Final answer:
Gem-bearing pegmatites are rare because they require unique conditions involving a high concentration of rare elements, the presence of water during formation, and a rapid drop in pressure, which do not commonly occur together. Additionally, these processes often take place deep within the Earth's crust, making accessibility and recovery challenging.
Step-by-step explanation:
Gem-bearing pegmatites are considered rare due to the unique conditions required for their formation. Pegmatites are igneous rocks that are formed during the last stages of crystallization of a magma chamber. They contain large crystals and are often the source of gemstones and rare minerals.
For a pegmatite to become gem-bearing, it must undergo a specific set of physical and chemical conditions that facilitate the formation of gem-quality crystals. These conditions include a high concentration of rare elements, the presence of water during the rock’s formation which lowers the temperature and allows for bigger crystal growth, and a rapid drop in pressure which assists in the formation of open spaces or cavities where crystals can develop freely.
However, the occurrence of all these factors together is rare, making gem-bearing pegmatites a scarcity. The rarity is also due to the fact that many of these conditions occur deep within the Earth's crust, in regions that are not easily accessible, such as those composed of ultramafic rock like peridotite. Furthermore, even when such conditions do occur and gemstones form, the geological processes may not bring them close enough to the Earth's surface, or erosion and weathering may render them unrecoverable.